Mountain biking is a widespread outdoor recreational activity and a competitive sport. In spite of this, few scientific studies have investigated the physiological aspects of mountain biking. Cross-country circuit race is the most popular mountain bike competitive event and it has been included as official Olympic sport in the 1996 Atlanta Summer Games. Cross-country is a mass-start endurance competition consisting in completing several laps of an off-road circuit characterized by repeated climbs and descents on gravel roads and field trails. Off-road cyclists usually use bikes with front suspension to decrease muscular stress on arms and legs as several isometric muscle contractions are necessary to absorb shock caused by rough terrain conditions, and bike handling and stabilization. Off-road cycling studies have shown that cross-country circuit races are performed at very high exercise intensity with average heart rate close to 90% of maximum corresponding to 84 ± 3 % of VO2max for races lasting up to two hours1, 2. The high intensity reported for cross-country competition could be related to race pace characteristic (e.g. fast start), the several climbs that require off-road cyclists to spend most part of their effort against gravity force and presumably greater rolling resistance, and the intense and repeated isometric contractions of arm and leg muscles. During cross-country races peak power output can reach values of 250 to 500 W during uphill cycling. Due to the high power output required during steep climbing, at the start of the race and when sprinting to pass slower riders, several authors suggested that high anaerobic power and/or capacity, other than high aerobic fitness, are likely to play an important role in off-road cycling competitions3. However, there is no scientific evidence for this and specific studies are necessary. The mountain bikers physiological characteristics indicate that a high aerobic power (VO2max 70 ml/kg/min) is a prerequisite to compete at high level in off-road events. Their relative VO2max and body mass are only slightly higher than the values reported in literature for climbers road cyclists. Some studies have also demonstrated that body mass should be take into account when evaluating physiological characteristics of mountain bikers and their relationships with off-road cycling performance. In a study by Impellizzeri et al.4 maximal and submaximal parameters of aerobic fitness explained 80% of the variance in off-road performance in a group of heterogeneous mountain bikers. However, more recently we have shown that the only parameter found correlated to a cross-country circuit race, in high level riders, was the intensity at respiratory compensation threshold and this explained only 40% of variance in performance (BJSM, in press). The unexplained 60% of variance in high-level off-road cycling performance could be related to other physiological factors not yet investigated such as the anaerobic energy system, non-respiratory buffering capacity (bicarbonate and non-bicarbonate) and the ability to recover during the descents following the several climbs. Furthermore, the technical ability could be an important determinant especially among mountain bikers characterized by homogeneous performance and physiological characteristics.